Optical image amplifier utilizing electron avalanches in a gas



July 22, 1969 w, M|LLER ET AL 3,457,418

OPTICAL IMAGE AMPLIFIER UTILIZING ELECTRON AVALANCHES 1N A GAS Filed Dec. 28, 1967 POWER SUPPLY g ze I a 3 W1 3 m a 0 1 N k I 8 I 2 Q v m w w o yl/I/II/I' INVENTORS HAROLD W. MILLER QUENTIN A. KERNS ,KAML 4401 A TTOR NEY POWER SUPPLY+ United States Patent Office 3,457,418 Patented July 22, 1969 U.S. Cl. 250-213 8 Claims ABSTRACT OF THE DISCLOSURE An intensifier for amplifying a very low intensity light image to a usable level operates by utilizing controlled electron avalanches. A very short voltage pulse is applied between a pair of spaced apart plates constructed as part of a waveguide, one of the plates being a photocathode. Electron avalanches are initiated between the plates by free electrons released from the photocathode as a result of an image projected thereon.

BACKGROUND OF THE INVENTION This invention relates generally to apparatus for detecting very low intensity light and more particularly to a very sensitive image intensifier.

The invention described herein was made in the course of, or under, Contract W7405eng-48 with the Atomic Energy Commission.

In the usual type of image intensifier a light image on a photocathode causes electrons to be released, the electrons then being accelerated toward an electron multiplying electrode. Each electron striking such electrode releases additional secondary electrons so that the number of electrons released as a result of an image is multiplied. However, the operation of the present invention is more related to that of one type of detector for charged particles and gamma and X-rays wherein a potential difference is provided between two spaced apart, parallel cathode and anode electrodes which are in the form of fiat plates or screens. The cathode is made sensitive to charged particle radiation and is provided with the more negative potential. An electron released from the cathode as a result of incident radiation initiates a discharge between the cathode and anode, the light resulting from the discharge having much higher energy than the initiating radiation. The spatial location of gamma and X-rays can thereby be determined in two dimensions. It is necessary that the potential between the electrodes be applied in a short pulse, otherwise the discharge very rapidly grows, destroying image definition and damaging the electrodes. However, utilization of discharges to obtain light image amplification has heretofore been unsatisfactory in that light from the discharge illuminates the photosensitive cathode and causes further discharges, thus destroying image resolution. Even in the intended operating condition for detecting gamma-ray or X-ray detection, fine resolution may not be obtainable owing to the spreading of the cross-sectional area of the discharge as it crosses from cathode to anode.

SUMMARY OF THE INVENTION The present invention utilizes an improved design which overcomes many of the above-discussed difiiculties. The problems concerned with light feedback to a photocathode have been eliminated and high image resolution is attained. A sufiiciently high degree of amplification is obtained so that a single photoelectron leaving a photocathode is detectable, an optical gain of more than 2-10 being obtainable. In the amplifier, a gas filled chamber is i defined by a pair of conductive plates with one plate being a photocathode of the type releasing one or more electrons in response to light. A high voltage pulse applied across the two plates has a magnitude sufiicient to create an electron avalanche. Ordinarily, such an avalanche would lead to creation of an are between the two plates, but in the present apparatus the high voltage is applied in a very short pulse so that the multiplication effect of an electron avalanche is obtained without arcing. Owing to the very short duration of the pulse, the voltage between the plates is removed before the avalanche matures into an are. If arcing were to occur, image resolu tion would be degraded and the plates would be damaged. Also, the deleterious efiect of light feedback to the photocathode is eliminated by utilizing a short pulse. A short time interval passes between the time an avalanche starts and the time light is first produced. By using a novel waveguide configuration, a voltage pulse with a duration shorter than such time interval can be provided so that the intensifier is insensitive to the light feedback. To impress such a short pulse between the plates, the chamber is constructed as part of a parallel plate or strip type transmission line with the same characteristic impedance as the pulse generator.

An image detector according to the present invention thus utilizes a fast pulse generator having a characteristic output impedance, a pair of spaced apart plates forming a strip transmission line having a characteristic impedance equal to the output impedance of the pulse generator, a transparent window disposed in one of the plates, a photosensitive material disposed in the space between the plates to intercept light passing through the window, coupling means connecting from the pulse generator to one end of the strip line formed by the plates, means terminating the other end of the strip line in the characteristic impedance, and means maintaining a gas between the plates.

It is an object of the present invention to provide an improved image intensifier having high definition.

It is another object of the present invention to provide an improved means for applying very fast voltage pulses across the electrodes of image intensifier.

It is another object of the present invention to provide an image intensifier with means for avoiding the deleterious effect of light feedback to a photosensitive element.

BRIEF DESCRIPTION OF THE DRAWING The invention will be best understood by reference to the following description together with the drawing of which:

FIGURE 1 is a section view of an image intensifier according to the invention with accompanying electrical circuits, and

FIGURE 2 is a section view of the image sensing region of the intensifier.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to the drawing, there is shown an image intensifier 11 for which a high voltage power supply 12 is provided. A coaxial transmission line 13 has a center conductor 14 which is coupled through a resistor 16 to a negative voltage terminal on power supply 12. The resistance of resistor 16 is high with respect to the characteristic impedance of line 13. The outer shield conductor 17 of the coaxial line is connected to a positive terminal on power supply 12. The length of line 13 will depend on the pulse length desired, for a typical pulse length of 8-10 second, the line length will be approximately 4 feet.

The image intensifier 11 has two spaced apart plates or electrodes 18 and 19 arranged in the form of a strip transmission line with an impedance equal to that of coaxial line 13. Plate 18 has a central window 21 with a coating 25 of photosensitive material thereon, as will be described in more detail later. A central portion 29 of the region between the plates 18 and 19 is enclosed by insulators 21) to contain one of the noble gases such as helium, neon, argon, etc., or mixtures of such gases, It is necessary in the operation of the intensifier to apply the short voltage pulse across the two plates 18 and 19 with a minimum of electrical refiections. Therefore, transition conductors are provided to connect the plates 18 and 19 to the coaxial line 13. One end of a sheet 22 of conductive material is fiat and is connected to one end of plate 19. The opposite end of sheet 22 is curved to conform to the shape of the shield 17 and makes electrical contact around a portion of the shield, the sheet 22 providing a gradual transition between the curved end of shield 17 and the end of fiat plate 19. A fast closing, normally open switch 23 such as a triggered spark gap is connected between the center conductor 14 and the narrow end of a cuneiform or delta shaped conductor 24. The wide end of the cuneiform conductor is connected to the plate 18. while the electrical transition provided by conductors 22 and 24 is not perfect, it provides sufiiciently constant impedance to avoid any significant reflections of the pulses.

The strip line formed by plates 18 and 19 is terminated in the characteristic impedance by a distributed resistance across the end of the line. Such a resistance can be approximated by a plurality of resistors 26 each connected in series with a DC. storage capacitor 27 between the ends of plates 18 and 19 opposite the transition conductors 22 and 24. The resistors 26 distribute the termination impedance fairly equally across the end of the strip line plates 18 and 19 and avoid creating undue reflections of the pulses applied across the image intensifier. The capacitance of capacitors 27 is suificiently high so that the reactance is insignificant with regard to the fast pulses. A direct current power supply 28 is connected across capacitor 27 and provides a steady state clearing potential between the plates 18 and 19 with the more positive potential being applied to plate 18. In the absence of a fast pulse from the line 13, the potential of the capacitors 27 and the power supply 28 is applied through the resistors 26 to the plates 18 and 19. An electric clearing field is applied thereby across tht plates 18 and 19 in a reverse direction to the field of the fast pulse. The resistors 26 isolate the potentials on the power supply 28 and capacitors 27 from the plates 18 and 19 during the occurrence of a fast pulse; while during fast pulse interpulse periods, the reverse field clears the region between the plates of free electrons as may be created by stray emission from the surface of the photosensitive coating 25. A lens 30 may be used to focus images onto the photosensitive coating 25.

The operation of the invention is now considered, with voltage and physical dimensions being given for a typical embodiment. The clearing potential between plates 18 and 19 is normally about twenty volts for a plate gap of one centimeter. Through resistor 16, a high potential from power supply 12 is applied between center conductor 14 and shield 17 of coaxial line 13. When switch 23 is closed, the stored energy in the coaxial line 13 is applied as a very fast pulse between plates 18 and 19, the length of line 13 determining the duration of the pulse. The conductivity of the photosensitive material 25 is preferably as high as possible to avoid producing reflections of the pulse, that is, the conductivity of the photosensitive material should be as close to that of the plate 18 as possible.

Refering particularly to FIGURE 2, there is indicated the position of an electron 31 after having been released from photosensitive material 25 as a result of light projected through window 21. The desired electric field intensity produced between plate 18 and plate 19 when switch 23 is closed may typically be kilovolts per centimeter. The high voltage gradient causes electrons released from material to accelerate toward plate 19. Each electron 4 produces electron-ion pairs in the gas in region 29, the electrons from each such pair in turn being accelerated toward plate 19 and producing further electron-ion pairs. The number of electron-ion pairs increases very rapidly, an avalanche 32 being formed in a tear-drop shape as shown in FIGURE 2. However, the short duration of the applied pulse allows the avalanche to grow to only a small size. Furthermore, the pulse duration is shorter than the recombination time of the electron-ion pairs in which the desired intensified light is produced. The image would be blurred or oblterated if the recombination light occured before the end of the pulse, since the recombination light would cause additional electrons to be released from the photosensitive material 25, each of which would then in turn start an avalanche with further production of light. However, in the novel construction of the present invention, such image degradation is avoided by providing the waveguide type configuration by which the very short voltage pulses are made possible. The short duration of the pulse also limits the size of avalanche 32 so that a high image definition is maintained. A typical dimension for the size of the avalanche, which determines the size of the image produced by each electron from photosensitive material 25, is 0.1 centimeter long and 0.1 millimeter in width. While only a single avalanche is shown in FIGURE 2, in ordinary usage electrons are released from many portions of material 25 according to the incident image.

Light produced by a single electron can be seen with the unaided eye, thus a very dim image can be greatly amplified. It should be further noted that a complete image is produced during each pulse. Each pulse, then, might be compared to one frame of a motion picture film and, by providing suitable synchronization between the shutter of a motion picture camera and the switch 23, a rapidly changing image can be intensified and recorded. If the photosensitive material is transparent then the intensified image can be viewed through window 21. However, a viewing window could be provided in plate 19. Such a window should, of course, have a high electrical conductance to preserve the electrical characteristic of the strip line 11.

As another variation, the plate 19 might be made transparent and the image to be intensified could be projected through plate 19 and region 29 to the photosensitive material. The resulting image could be observed either through plate 19, or through plate 18 if the photosensitive material is transparent.

What is claimed is:

1. In an image intensifier, the combination comprising a fast pulse generator having a characteristic output impedance, a pair of spaced apart equally conductive plates forming a strip transmission line having a characteristic impedance equal to the output impedance of said generator, a transparent window disposed in at least a first of said plates, a photosensitive material coating said window and disposed in the space between said plates to intercept light passing through said window, said photosensitive material having a conductance that is substantially equal to the conductance of said plates, coupling means having a characteristic impedance equal to said generator output impedance and said strip line impedance, said coupling means connecting said generator to said plates at one end of said strip line formed thereby, means terminating the other end of said strip line in said characteristic impedance, means for maintaining a gas between said plates, said gas being responsive to a coincidence of an electron emission from said photosensitive material and the occurrence of an electric field between said plates to form an electron-ion avalanche, and switch means for applying a pulse from said generator to said plates through said coupling means for producing an electric field therebetween, said pulse having a period less than the arc formation time of an electron-ion avalanche between said plates.

2. An image intensifier as described in claim 1 wherein said photosensitive material has a high conductivity and is disposed on the inside surface of said window, said material being electrically connected to said first plate, said pulse generator having a negative potential terminal coupled to said first plate.

3. An image intensifier as described in claim 1 wherein there is provided in said fast pulse generator a transmission line having at least two conductors, a direct current power supply having two output voltage terminals, one of said terminals being connected to one of said conductors in said transmission line, an element having a high impedance compared to said characteristic impedance and connected from the other of said terminals of said power supply to the other one of said conductors of said transmission line, and said switch means is connected between one of said conductors in said transmission line through said coupling means to one of said plates in said strip line and, the other of said conductors in said transmission line is connected through said coupling means to the other one of said plates in said strip line.

4. An image intensifier as described in claim 3 wherein said transmission line is a coaxial type line and wherein said coupling means utilizes thin broad conductive elements, one being tapered in width from the dimension of the center conductor of said coaxial line to the width of one of said plates and, the other of said conductive elements at one end being conformed to the shape of a portion of the outer conductor of said coaxial line and being gradually flattened toward the other end to conform to the shape of the other one of said plates, whereby the characteristic impedance of said coupling means generally equals that of said transmission line and of said strip line formed by said plates.

5. An image intensifier as described in claim 1 wherein the constituents of said gas come only from the noble gas group.

6. An image intensifier as described in claim 1 wherein said means maintaining a gas is an insulative annulus disposed between said plates and in sealing contact therewith.

7. An image intensifier as described in claim 1 wherein said termination means is a plurality of resistors connected at spaced apart positions across the end of said strip line formed by said plates at the end of said line opposite said coupling means.

8. An image intensifier as described in claim 7 further including a direct current power source for establishing a clearing field between said plates, a plurality of capacitors each serially connected with a respective one of said resistors across the end of said strip line opposite said coupling means, and means connecting said clearing field power source across said capacitors to charge each of said capacitors, said capacitors being charged to a potential level having a polarity that is opposite to the polarity of fast pulses from said generator.

References Cited UNITED STATES PATENTS 2,485,930 10/1949 Silliman 33334 X 2,709,242 6/1955 Reed 333--34 X 2,860,308 11/1958 Bales 33310 2,929,949 3/ 1960 Vincent.

3,310,678 3/1967 Kylander et al. 250-213 X 3,337,733 8/1967 Charpak et a1. 313- X RALPH G. NILSON, Primary Examiner C. M. LEEDOM, Assistant Examiner Us. 01. X.R. 

